In 2017, researchers developed a new formula to calculate gestational age during pregnancy that is more accurate than an existing formula used in clinical practice since 1984, (Skupski et al. Obstetrics and Gynecology, 2017). Ultrasound biometric measurements of the fetus are entered into a calculator to estimate gestational age, a process that improves upon estimating gestational age from the reported date of last menstrual period. The study provides specific numbers of days difference between gestational age by ultrasonography and the reported last menstrual date. These data can guide recommendations for when pregnancies should be redated based on ultrasound measurements. Using the NICHD formula, 25% of women would have more accurate dating, thus preventing interventions for preterm or postterm pregnancy. Up to 25% of the US population undergoes labor induction, providing a significant potential to prevent morbidity from unnecessary interventions. Researchers also found that greater burden of either stress or depressive symptoms during pregnancy were not associated with alterations in overall fetal weight or individual biometric parameters. (Grobman et al. Ultrasound Med. 2017) Additionally, researchers found that neonatal biometric measures did not vary by Cohens Perceived Stress Survey class (Wing et al. AJOG. 2017). Further, researchers found that weight loss or excessive weight gain in the first trimester were not associated with adverse birthweight outcomes, though in the 2nd and 3rd trimester gestational weight gain (GWG) above or below the reference trajectory was associated with small (SGA) or large for gestational age (LGA; Pugh et al. AJOG. 2017). Obese Cohort Obesity is common among women of reproductive age and is known to increase the risk for maternal and fetal pregnancy complications. The NICHD Fetal Growth Studies enrolled 468 obese women with singleton pregnancies with the goal of comparing fetal growth patterns between women with obesity and non-obese women. Furthermore, because pregnancy complications such as GDM and preeclampsia are more common in women with obesity, this additional cohort offers the opportunity to examine how fetal growth is impacted by such complications. The researchers found that as early as 32 weeks' gestation, fetuses of obese women had higher weights than fetuses of nonobese women (Zhang et al. JAMA Pediatrics 2018). Dichorionic Twin Cohort Twin gestations represented 3.4% of U.S. births in 2013, yet there is limited contemporary data on the estimation of fetal growth trajectories in twins. The NICHD Fetal Growth Studies enrolled 171 dichorionic twin pregnancies. The primary objective was to empirically define the trajectory of fetal growth in dichorionic twins using longitudinal two-dimensional ultrasonography and to compare the fetal growth trajectories for dichorionic twins with those based on a growth standard developed by our group for singletons. Additional research is focusing on the influences of maternal and pregnancy characteristics on fetal growth. Twin pregnancies put additional demands on maternal nutritional status due to the increased maternal and fetal tissue mass. However, the current gestational weight gain recommendations for twin pregnancies are provisional due to limited research specific to twins. In 2017, we studied the associations between maternal weight gain across pregnancy and the growth of the twins as determined by ultrasonography (Hinkle et al. American Journal of Clinical Nutrition 2017). We found that maternal weight gain in the second trimester was associated with fetal growth of the twins. Specifically, the findings were driven by an association with the abdominal circumference earlier in second trimester and the long bones (femur and humerus length) later in the second trimester. The larger intrauterine size persisted to delivery, which was demonstrated by the significant association between maternal weight gain in the second trimester and birth weight. These findings should help to direct intervention studies to determine whether modification of a womans weight gain trajectory can enhance fetal growth and pregnancy outcomes in women with dichorionic twin pregnancies. Biomedical Markers in Relation to Gestational Diabetes and Fetal Growth The NICHD Fetal Growth Studies is the basis for studying the pathogenesis of gestational diabetes (GDM) and fetal growth and to identify factors that can improve early prediction of GDM. This work is grounded within an evolving body of research suggestive of important roles of maternal metabolism and nutrition in the development of GDM and in fetal growth. Pathway specific biomedical markers, and non-targeted metabolomics and lipidomics were measured longitudinally in 107 GDM cases and 214 non-GDM controls in the NICHD Fetal Growth Studies-Singleton Cohort (c.f. Gestational Diabetes Mellitus: Epidemiology, Etiology, and Health Consequences). Our recent findings suggest that elevated iron stores may be involved in the development of GDM from as early as the first trimester (Rawal et al. Diabetologia 2017). Following this finding, we recently completed an invited review which supports a potential link between greater iron intakes, body iron stores or status before or during pregnancy and an elevated GDM risk (Zhang et al. American Journal of Clinical Nutrition 2017). In addition, recent work has focused on hormone levels and GDM risk such as our work providing evidence that thyroid function early in pregnancy may be an indicator for subsequent risk of GDM (Rawal et al. Journal of Clinical Endocrinology & Metabolism, 2018). We identified novel lipid species associated with GDM risk and observed different association pattern by lipid class, acyl chain carbon number and double bond number, and the timing of lipid measurement during pregnancy (Zhu et al. AM J Clin Nutrition, 2018). Lastly, we recently published findings suggesting a potential important clinical utility of HbA1c measurement in the first trimester of pregnancy, even among low-risk women (Hinkle et al. Scientific reports, 2018). While our findings require replication, GDM prediction was significantly improved with the inclusion of HbA1c over conventional risk factors suggesting that it could be used to improve early risk-stratification and screening in women with elevated levels. Analyses on additional biomarkers on both GDM and fetal growth are ongoing.